The present invention relates to a backstay spreader used on a crane, such as mobile lifting crane, and particularly a backstay spreader that can have its length adjusted after being interconnected between a pair of backstay straps.
Lift cranes typically include a carbody; ground engaging members elevating the carbody off the ground; a rotating bed rotatably connected to the carbody such that the rotating bed can swing with respect to the ground engaging members; and a boom pivotally mounted on the rotating bed, with a load hoist line extending there from. For mobile lift cranes, there are different types of moveable ground engaging members, most notably tires for truck mounted cranes, and crawlers. Typically mobile lift cranes include a counterweight to help balance the crane when the crane lifts a load.
Since the crane will be used in various locations, it needs to be designed so that it can be transported from one job site to the next. This usually requires that the crane be dismantled into components that are of a size and weight that they can be transported by truck within highway transportation limits. The ease with which the crane can be dismantled and set up has an impact on the total cost of using the crane. Thus, to the extent that fewer man-hours are needed to set up the crane, there is a direct advantage to the crane owner or renter.
When the crane needs to work on particularly high buildings or structures, or in restricted spaces, a jib may be mounted at the top of the boom to provide required reach. This could be a fixed jib or a luffing jib. When a jib is employed, one or more jib struts are connected to the top of the boom or bottom of the jib. These struts support the jib rigging and provide a moment arm about which force can be applied to support a load being lifted by the jib. When a luffing jib is used, frequently two struts will be used, and the angle between the struts will be controlled by jib hoist rigging. Changing the angle between the two struts will thus change the angle between the boom and the luffing jib.
When a jib is used on a crane, jib backstay straps are connected between the jib strut and the rotating bed, typically by being connected to the boom butt, which of course is pivotally connected to the rotating bed. These straps are made of several separate sections. The boom also has straps (which could be referred to as backstay straps) associated with it, which connect between the top of the boom and either an equalizer suspended between the boom and a fixed mast, or between the boom and the top of a live mast. On a typical crane with a fixed mast, the boom hoist rigging comprises multiple parts of line that run between the equalizer and the top of the mast, and is used to control the angle of the boom.
The jib backstay straps and the boom hoist rigging potentially interfere with one another, since the boom hoist rigging and boom straps go between the top of the mast and the top of the boom, and the jib backstay straps go between the bottom of the boom and the strut supporting the jib, which extends backward from the top of the boom. If the boom and boom hoist rigging are very large, the jib backstay straps need to be spread apart to give room so that the jib backstay straps do not come into contact with the parts of line of the boom hoist rigging or the boom straps. The need for spreading of the jib backstay straps is even greater considering that not only does contact need to be avoided when the boom and jib are static, but when there is a load on the hook and the boom swings, bending and twisting moments in the boom and jib strut can cause deflection in the rigging that would cause contact if the jib backstay straps were not spread far enough apart.
It is convenient to transport the sections of the boom straps and jib backstay straps with the sections of boom between one job site and the next. This is because, for the most part, the number of sections and the length of each section of the boom straps and the jib backstay straps that will be needed are dependent on the number and lengths of the sections that are used to construct the boom. For example, a 100 foot boom may be made from a 10 foot boom butt, a 10 foot boom top and four 20 foot boom inserts. However, if the boom is going to be 120 feet long, five 20 foot boom inserts will be used. If the boom is going to be 130 feet long, five 20 foot inserts and one 10 foot insert will be used. For each of these different boom configurations, different numbers and lengths of sections of the boom straps and jib backstay straps will be used. However, it is convenient if the straps are transported between job sites on the tops of the boom sections where they can be easily connected together to make up the boom straps and jib backstay straps when the sections of boom are put together.
One problem that is encountered when a high-capacity crane is transported is that the width of the boom sections is limited by highway transportation limits. However, when the crane is set up, the equalizer may be so wide that the jib backstay straps will need to be further apart during use than the width at the points where the backstay straps are connected to the boom butt and strut top. In that case a spreader may be needed to spread the jib backstay straps apart once the crane is set up. While the sections of the jib backstay straps can still be transported on the top of the boom sections, they will need to be wider apart during use in the area of the boom hoist rigging than in their transport position.
One problem in providing a spreader between the jib backstay straps is that the spreader needs to be in place during operation, but where it needs to be placed is very high off the ground when the boom, mast, luffing jib and struts are in their operational position. Putting a spreader in place before the jib backstay straps are raised into place is not possible if the strap sections are sitting in brackets on the top of the boom sections on which they were transported. Thus there is a need for a way to spread the jib backstay straps apart after they have been raised up to the point that they are no longer in the brackets on the top of the boom sections. However, once they are raised up it is very difficult to position a worker at the elevation where the spreader needs to go and have that worker be able to insert a spreader between the jib backstay straps and spread the straps apart.